We develop and test models for the population dynamics of species that undergo regular alternations of generations between independent, free-living, haploid and diploid phases. The models are patterned after the dioecious, haploid-diploid lifecycle of many marine algae. If the two phases have equal demographic rates, all models (with or without density dependence) predict a stable distribution with a ratio of root2:1 (similar to60% haploids and 40% diploids). We find that observable deviations from this distribution can occur when the demographic rates (mortality, fecundity) between phases vary widely. Field surveys of three macroalgal species with independent, free-living haploids and diploids, Mazzaella flaccida, M. laminarioides, and M. splendens, reveal a consistent pattern of haploid dominance that significantly exceeds this null model expectation (72%, 76%, and 66%, respectively). These species also exhibit significant variation in haploid frequency among sites. We estimated the per capita fecundity and mortality rates for each phase of both M. flaccida and M. laminarioides and used this information to address two basic questions: (1) Which parameter better explains the overall high relative haploid abundance for each species? and (2) Which parameter better explains the variation in relative haploid abundance among sites? Differences in haploid and diploid mortality rates did not explain the high relative abundance of haploids for either M. flaccidet or M. laminarioides. However, differences in fecundity rates between phases were consistent with the high relative abundance of haploids for M. flaccida; diploids usually had significantly higher per capita fecundity rates than haploids. For M. laminarioides, there were no significant differences in fecundity rates between phases. For M. flaccida, among-site variation in relative abundance of haploids can be explained by differences in mortality rates. The relative abundance of haploids was positively correlated with the ratio of diploid:haploid mortality rates; the sites with the highest ratio of diploid:haploid mortality had the highest percentage of haploids. Differences in mortality rates for M. laminarioides, and fecundity rates for M. flaccida and M. laminarioides, could not explain the observed variation in haploid abundance among sites.